Accumulation returns n-times more value than actual number of summed values present. The factor n is dependent upon the number of times the parent object updates are called before rule that does the summation is invoked in the flow.
Fixed by https://github.com/droolsjbpm/drools/commit/fa36f1cbe
This is related to the fix linked, so we need to include it in 6.0.2 as well.
Cherry-picked to 6.0.x branch with https://github.com/droolsjbpm/drools/commit/2455c628f
Verified in BRMS 6.0.2.CR1. Already covered by community unit tests which are regularly run against product build.